For example, in order to cool superconductors operated at a temperature of not higher than 100K, a refrigerating machine such as a pulse tube refrigerating machine or a sterling refrigerating machine is used. For example, JP-A-2001-144635 discloses cooling of a wireless receiving unit by using a pulse tube refrigerating machine. This wireless receiving unit includes a receiving band filter and a low noise receiving amplifier. Further, according to the technique disclosed in JP-A'635, a Peltier element is fixed to the refrigerating machine, and the receiving band filter and the low noise receiving amplifier are fixed to Peltier element, so that the wireless receiving unit can be further cooled to a temperature lower than the temperature generated by the refrigerating machine. Thus, it is possible to remove the heat from the wireless receiving unit and operate the wireless receiving unit at low temperatures without increasing the cooling capacity of the refrigerating machine.
Recently, there is a demand that the temperature of a circuit device including a superconductor is lowered and also the low temperature is precisely controlled. Especially, when two or more electronic devices or electronic units are contained in one circuit device, there is a demand that the electronic devices and electronic units are cooled to temperatures which are different from and close to each other.
To satisfy the above demand, it is necessary to use a multiple stage refrigerating machine or two or more refrigerating machines. For example, when a two stage type refrigerating machine is used, it is necessary in a vacuum space of a cryostat that a cooling end (cold head) of the first stage is set at a temperature of about 20K and a cooling end (cold head) of the second stage is set at a temperature of about 70K, and also a first article to be cooled is arranged in the first cold head and a second article to be cooled is arranged in the second cold head. A temperature sensor and heater are provided when necessary, and the wirings of the temperature sensor and heater are drawn from the vacuum container to connect them to a control unit arranged outside the vacuum container. The temperatures of the first and second article to be cooled are respectively controlled to a desired temperature, accordingly.
When two or more refrigerating machines are used, the number of the refrigerating machines is selected to be the same as that of the articles to be cooled, and the articles are cooled by the respective refrigerating machines. In this method, as in the multiple stage type refrigerating machine described above, a temperature sensor and heater are arranged when necessary, and temperatures of the articles are respectively controlled to a desired temperature.
However, according to the methods described above, since two or more articles to be cooled have to be cooled to different temperatures, it is necessary to use a refrigerating machine having the complicated structure, and also to use a plurality of refrigerating machines, thereby making the entire structure complicated, along with extension of a space for the cryostat. Further, when it is desired that a plurality of articles to be cooled are located close to each other, many problems tend to occur. Furthermore, even when a necessary difference between the cooling temperatures is a small amount of about 5 to 30K, a cooling device having the complicated structure must be used, and thus the articles to be cooled must be arranged under the restricted conditions.